Field of the Invention
The present invention relates to a tightening structure for a cylinder head bolt, which is used in fastening or tightening a cylinder head bolt, inserted through a cylinder head, into either a cylinder or a crankcase, both forming respective parts of an engine casing, to thereby firmly connect the cylinder head with the engine casing.
Description of Related Art
For example, In the assembly line of combustion engines for use in motorcycles, as shown in FIG. 1, when a cylinder head 1 is connected with an engine casing 2 including a cylinder 3 and a crankcase 4, a relatively elongated cylinder head bolt 8 is, after having been passed through an insertion hole 1a defined in the cylinder head 1 and then through a insertion hole 3a defined in the cylinder 3, threaded into a threaded hole 4a defined in the crankcase 4. By so doing, the cylinder head 1 and the crankcase 4 are connected together with the cylinder 3 interposed between the cylinder head 1 and the crankcase 4. In this connection, see the patent document 1 listed below.
As the cylinder head bolt 8 generally used in such a tightening structure as discussed above, as shown in FIG. 5 showing the conventional tightening structure, a tension bolt is utilized. The tension bolt 8 has at an upper end portion a rotation operating part 8a and at a lower end portion a male threaded region (shown in FIG. 1). Further, the tension bolt has a columnar part 8b situated intermediate between the upper and lower end portions. The outer diameter R2 of the columnar part 8b is smaller than the outer diameter R1 of the upper end portion and the outer diameter of the lower end portion. When this cylinder head bolt 8 in the form of the tension bolt discussed above is fastened, a large elongation is generated in the intermediate columnar part 8b of the smaller outer diameter. An axially acting force of a significant tension is brought about by the restoring force induced upon generation of such an elongation. The cylinder head and an engine casing 2 (shown in FIG. 1) is firmly connected together by such an axially acting restoring force. It is to be noted that a plain washer 9 is employed under a head of the cylinder head bolt 8.
Step-by-step measurement of the axially acting force generated in the cylinder head bolt 8 during the assemblage of the automotive engine results in increase of hours of work. Accordingly, the axial force of the cylinder head bolt 8 is controlled by the tightening torque that can be directly measured with a torque wrench for tightening the cylinder head bolt 8.